This invention relates to an anode positioning system for an electrolytic cell having multiple suspended anodes, and specifically to a system for raising and lowering such anodes.
An important particular application of the invention is in the production of aluminum metal in a cell of the well-known type containing a multiplicity of prebaked, block-shaped carbon anodes individually suspended from a superstructure so as to be simultaneously in contact with the electrolyte of the cell. Alumina dissolved in the molten salt electrolyte is reduced by passage of direct current therethrough, between the anodes and a pool of the produced molten metal that collects at the bottom of the cell in contact with a carbon cell lining which is connected electrically to enable the pool to serve as the cell cathode. As cell operation proceeds, the molten metal level in the cell varies, owing to the progressive accumulation and periodic removal of product metal. Since cell efficiency is dependent on the anode-cathode distance, i.e. the vertical spacing between the anodes and the subjacent electrolyte-metal interface, it is necessary to adjust the vertical position of the anodes in correspondence with these changes in metal level. Other circumstances also require vertical movement of anodes, either collectively or individually, from time to time; for example, as the carbon anode blocks become consumed by oxygen liberated at the anodes, their remnants must eventually be raised out of the cell and replaced. In addition, agitation of the cell bath is sometimes desirable for the purpose of quenching so-called anode effects. Such agitation is commonly effected, in present-day practice, by inserting a piece of green wood into the molten electrolyte, thereby to cause gases and vapors to bubble through the electrolyte as the wood is violently consumed.
It is already known to mount each anode of a multi-anode cell on its own individual screw jack, each jack having a pneumatic or electric motor which drives the jack (to raise or lower the associated anode). A variety of problems has been encountered in the provision and operation of these devices. Not only do they require relatively complex, large and costly components, but in addition they are vulnerable to development of excessive torque from jamming or other causes, that can result in motor burnout or damage to other components. Moreover, in the operation of such devices, it is difficult to achieve desired uniformity of displacement of plural anodes. Other anode jacking systems with individual screw jack devices incorporate clutches with interlocking teeth or jaws mounted on the output shafts of worm reduction gear boxes. When the clutches are disengaged, the anodes tend to descend from their own weight unless brakes are provided; and in the event of drive failure it is usually not feasible to move the anodes at all. The elimination or reduction of these problems would contribute advantageously to desired convenience, simplicity and economy in such operations as the electrolytic production of aluminum metal in multi-anode cells.